As a light-emitting element forming a display device such as a display unit or an illuminating device, there has been known an EL element utilizing a substance which self-emits light in the presence of an applied voltage due to an electroluminescence (EL) phenomenon. The EL element is a thin-film light-emitting element including a light-emitting layer which is provided between an upper electrode and a lower electrode and is made of an organic material or an inorganic material. The EL element has a structure such that the upper and lower electrodes apply a voltage to the light-emitting layer so as to cause light emission.
In recent years, there has been developed a light-emitting element having a resonator structure (what is referred to as a microcavity structure) in which light emitted in the light-emitting layer is resonated by using a total reflection mirror as one of the upper electrode and the lower electrode and using a semi-transmissive mirror that transmits light at some of wavelengths as the other electrode (see Patent Literature 1, for example).
Patent Literature 1 discloses a blue light-emitting element including a light-emitting layer formed from a light-emitting material having internal emission spectral peaks (a first peak and a second peak) in a blue wavelength range and a green to red wavelength range, respectively, and a resonator structure having a single resonance spectral peak in the blue wavelength range. The blue light-emitting element of Patent Literature 1 outputs the first and second peaks while strengthening the first peak in the blue wavelength range and weakening the second peak in the green to red wavelength range by the resonator structure.
The technique disclosed in Patent Literature 1 may be effective for a display device such as a large display which requires wide viewing angle characteristics. However, in a personally-used small display such as a mobile terminal, a personal computer, or a car navigation system, in some cases a variation in luminance in a front direction cannot be tolerated.
That is, in a case where the resonator structure is employed, a luminance in the front direction increases due to its filter characteristics and strong directional characteristics of an emission output. A personally-used display device which does not require wide viewing angle characteristics, for example, utilizes such directional characteristics. Therefore, as compared to a television set which requires a wide viewing angle, the personally-used display device requires a less variation in luminance in the front direction. In a thin-film light-emitting element having such a resonator structure, however, the filter characteristics thereof are sensitive to a distance between mirrors (a resonator optical path length), and in some cases chromatic coordinates (color purity) or a luminance fluctuation in the front direction cannot be tolerated if a variation occurs in the resonator optical path length due to a manufacturing error in the manufacturing process. In the case of the light-emitting element disclosed in Patent Literature 1, a luminance fluctuation due to an increase or a decrease in resonator optical path length is large. As a result, in some cases a variation in luminance in the front direction cannot be tolerated.